1. Field of the Invention
The present invention relates generally to an alkali storage battery such as a nickel-hydrogen storage battery, a hydrogen absorbing alloy electrode used as a negative electrode of the alkali storage battery, and a hydrogen absorbing alloy used for the hydrogen absorbing alloy electrode, and is particularly characterized in that the hydrogen absorbing alloy used for the hydrogen absorbing alloy electrode is improved, to improve discharge characteristics in the alkali storage battery using the hydrogen absorbing alloy electrode and particularly, discharge characteristics in a case where the alkali storage battery is discharged in a low-temperature environment and a case where the alkali storage battery is discharged at a high current as well as to prevent the internal pressure of the alkali storage battery from rising.
2. Description of the Related Art
A nickel-hydrogen storage battery has been conventionally known as one of alkali storage batteries. In the nickel-hydrogen storage battery, a hydrogen absorbing alloy electrode using a hydrogen absorbing alloy for its negative electrode has been used.
In recent years, such an alkali storage battery has been employed for a power supply of each type of portable equipment or the like. Therefore, it has been necessary to increase the capacity of the alkali storage battery.
In the conventional alkali storage battery, however, the specific surface area of the hydrogen absorbing alloy used for the hydrogen absorbing alloy electrode was generally small. In the hydrogen absorbing alloy electrode, the amount of hydrogen which can be absorbed and discharged was decreased. Accordingly, it was difficult for the alkali storage battery to have a high capacity.
Therefore, in recent years a hydrogen absorbing alloy electrode using hydrogen absorbing alloy powder having a particle diameter of not more than 25 xcexcm and having a large specific surface area to increase the amount of hydrogen which can be absorbed and discharged, to improve the capacity of an alkali storage battery has been proposed in JP-A-60-70665.
In the above-mentioned application, in fabricating the hydrogen absorbing alloy electrode, the hydrogen absorbing alloy powder is sintered at a high temperature of 950xc2x0 C.
When the hydrogen absorbing alloy powder is thus sintered at a high temperature of 950xc2x0 C., however, the hydrogen absorbing alloy is oxidized, so that the concentration of oxygen contained in the hydrogen absorbing alloy is increased. Consequently, the activity on the surface of the hydrogen absorbing alloy is decreased, so that an electrochemical reaction does not easily progress. Particularly, discharge characteristics in a case where the alkali storage battery is discharged under a low-temperature environment and a case where the alkali storage battery is discharged at a high current are degraded.
When the hydrogen absorbing alloy powder is sintered at the above-mentioned high temperature, a portion where hydrogen absorbing alloy particles are bonded together is made large and consequently, dense. Therefore, the specific surface area of the hydrogen absorbing alloy in the hydrogen absorbing alloy electrode is decreased, so that the capacity of the alkali storage battery is decreased. Further, gas absorption performance in the hydrogen absorbing alloy electrode is degraded. Oxygen gas produced inside the alkali storage battery when the battery is overdischarged, for example, cannot be sufficiently absorbed. Accordingly, the internal pressure of the battery rises.
A first object of the present invention is to improve, in a hydrogen absorbing alloy electrode used as a negative electrode of an alkali storage battery such as a nickel-hydrogen storage battery, the hydrogen absorbing alloy used for the hydrogen absorbing alloy electrode, to improve discharge characteristics in the alkali storage battery and particularly, discharge characteristics in a case where the alkali storage battery is discharged under a low-temperature environment and a case where the alkali storage battery is discharged at a high current.
A second object of the present invention is to improve gas absorbability in a hydrogen absorbing alloy electrode for an alkali storage battery, to prevent the internal pressure of the alkali storage battery from rising.
In a hydrogen absorbing alloy for an alkali storage battery in the present invention, hydrogen absorbing alloy powder containing not less than 50% by weight of particles having a particle diameter of not more than 25 xcexcm is sintered at a temperature of not more than 850xc2x0 C.
In a hydrogen absorbing alloy electrode for an alkali storage battery in the present invention, a hydrogen absorbing alloy obtained by sintering hydrogen absorbing alloy powder containing not less than 50% by weight of particles having a particle diameter of not more than 25 xcexcm at a temperature of not more than 850xc2x0 C., as described above, is used.
In an alkali storage battery according to the present invention, a hydrogen absorbing alloy electrode using a hydrogen absorbing alloy obtained by sintering hydrogen absorbing alloy powder containing not less than 50% by weight of particles having a particle diameter of not more than 25 xcexcm at a temperature of not more than 850xc2x0 C., as described above, is used as its negative electrode.
When the hydrogen absorbing alloy powder containing not less than 50% by weight of particles having a particle diameter of not more than 25 xcexcm is sintered at a temperature of not more than 850xc2x0 C., as in the present invention, the hydrogen absorbing alloy is prevented from being oxidized, so that the concentration of oxygen contained in the hydrogen absorbing alloy is decreased. Accordingly, the activity on the surface of the hydrogen absorbing alloy is prevented from being decreased. When the hydrogen absorbing alloy electrode using the hydrogen absorbing alloy is used as a negative electrode of the alkali storage battery, therefore, an electrochemical reaction in the hydrogen absorbing alloy electrode is accelerated. Particularly, the alkali storage battery has a sufficient discharge capacity even in a case where it is discharged under a low-temperature environment and a case where it is discharged at a high current, resulting in improved low-temperature discharge characteristics and high-rate discharge characteristics in the alkali storage battery. When the particle diameter of the hydrogen absorbing alloy powder is less than 1 xcexcm, it is difficult to handle the powder in processes. Accordingly, it is preferable to use hydrogen absorbing alloy powder having a particle diameter of not less than 1 xcexcm.
Furthermore, in the present invention, the hydrogen absorbing alloy powder is sintered at a temperature of not more than 850xc2x0 C., as described above. Accordingly, a portion where hydrogen absorbing alloy particles are bonded together is prevented from being made large and consequently, precise as in a case where the powder is sintered at a high temperature of 950xc2x0 C. Therefore, the specific surface area of the hydrogen absorbing alloy in the hydrogen absorbing alloy electrode is kept large, so that the alkali storage battery has a sufficient capacity. Further, the number of voids in the hydrogen absorbing alloy electrode is increased, so that gas absorption performance in the hydrogen absorbing alloy electrode is improved. Gas produced inside the alkali storage battery is sufficiently absorbed by the hydrogen absorbing alloy electrode. Accordingly, the internal pressure of the battery is prevented from rising.
In the present invention, in order to further reduce the concentration of oxygen contained in the hydrogen absorbing alloy, it is preferable to use, as the hydrogen absorbing alloy powder, one ground in an inert atmosphere. For example, it is preferable to use hydrogen absorbing alloy powder ground in an inert gas atmosphere such as argon and an inert solvent such as carbon tetrachloride, toluene, or xylene. Further, it is possible to use a known grinding method as a method of grinding the hydrogen absorbing alloy. For example, a ball mill, jet mill, or an agitation mill can be used.
As the type of hydrogen absorbing alloy used in the present invention, it is possible to use a known misch metal alloy or a Laves phase alloy which is generally used.
It is possible to use, as the misch metal alloy, one expressed by a constitutional formula of MmNiaCobAlcMd having a CaCu5 type crystal structure, for example. In the constitutional formula, Mm is a misch metal, and M is at least one element selected from Mn and cu. The ratio of atoms a, b, c, and d satisfies conditions of 3.0xe2x89xa6axe2x89xa65.2, 0.1xe2x89xa6bxe2x89xa61.2, 0.1xe2x89xa6cxe2x89xa60.9, 0.1xe2x89xa6dxe2x89xa60.8, and 4.4xe2x89xa6a+b+c+dxe2x89xa65.4.
It is possible to use, as the Laves phase alloy, one expressed by a constitutional formula of ZrpTi1-pNiqCorMs having a C14 or C15 type Laves phase structure, for example. In the constitutional formula, M is at least one element selected from Al, Mn, V, Mo, and Cr. The ratio of atoms p, q, r, and s satisfies conditions of 0xe2x89xa6pxe2x89xa61.0, 1.0xe2x89xa6qxe2x89xa62.0, 0.2xe2x89xa6rxe2x89xa61.0, 0.2xe2x89xa6sxe2x89xa60.8, and 1.8xe2x89xa6p+q+r+sxe2x89xa62.2.
In the alkali storage battery in the present invention, in order to further accelerate an electrochemical reaction in the hydrogen absorbing alloy electrode used as its negative electrode to further improve discharge characteristics in a case where the alkali storage battery is discharged under a low-temperature environment and a case where the alkali storage battery is discharged at a high current, it is preferable that the concentration of oxygen contained in the hydrogen absorbing alloy obtained by the sintering is not more than 0.30% by weight.
In the alkali storage battery in the present invention, in order to further improve gas absorption performance in the hydrogen absorbing alloy electrode used as its negative electrode, it is preferable that the specific surface area of the hydrogen absorbing alloy obtained by the sintering is not less than 0.030 m2/g.
There and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate specific embodiment of the invention.